This invention relates in general to bearing systems for mounting shafts in housings and, more particularly, to a bearing system that compensates for differential thermal expansion between a shaft and a housing.
Machines of a wide variety utilize rotating shafts to achieve the purposes for which they are designed. Typically, the shaft rotates in a housing on antifriction bearings—often tapered roller bearings arranged in pairs and in opposition so that the bearings not only transfer radial loads between the shaft and housing, but axial or thrust loads as well. The shafts and bearings are usually made from steel, but it is not unusual to find the housings made from a different metal, such as aluminum, with a higher coefficient of thermal expansion.
To give the steel shaft a measure of stability, so that its axis of rotation remains fixed with respect to the housing in which it rotates, the bearings should be set to light preload. This eliminates all clearances within the bearings themselves. Moreover, the races of the bearings should be installed over the shaft and within the housing with interference fits. This eliminates all clearances between the races and the shaft and housing. However, as the temperature of the shaft and housing rises, usually as a consequence of friction generated during operation, the housing grows more than the shaft and the bearings and the outer races may become loose in the housing. This destabilizes the axis of rotation.
The typical automotive differential for rear wheel drive vehicles certainly demonstrates the problem. It has an aluminum housing to save weight and a steel pinion shaft which rotates in the housing on two single row tapered roller bearings that are mounted in the indirect configuration, that is to say with the rollers of the two rows tapering downwardly toward each other. When the temperature of the differential rises, the housing expands more than the shaft and the cups (outer races) of the two bearings may become loose in the housing.